Power supply units of a server system usually operate in a 1+1 operation mode. In the 1+1 operation mode, two power supply units are connected in parallel, and both output at half power to the server system. When one of the two power supply units is down, the other power supply unit will output more power to ensure the server system operates normally. When the server system needs more power, the two power supply units should be operated in a 2+0 operation mode. In the 2+0 operation mode, the two power supply units are connected in series, and both output full power to the server system. However, the power supply units of the server system can only operate in the 1+1 operation mode or the 2+0 operation mode.